Among examples of the conventionally-known engines equipped with a secondary air supply device are ones in which an exhaust pipe is provided for directing exhaust gas to an exhaust muffler and in which a secondary air introduction pipe is provided in communication with the exhaust pipe so that air (secondary air) is introduced into the exhaust muffler via the secondary air introduction pipe using exhaust pulsation of the engine. By air being introduced into the exhaust muffler via the secondary air introduction pipe like this, an oxidation-reduction reaction (detoxifying reaction) can be caused in exhaust gas by means of a catalyst (catalytic) converter provided within the exhaust muffler.
Further, as a means for efficiently directing air into the exhaust muffler via the secondary air introduction pipe in such engines, it is required that a tailpipe connected to the exhaust muffler be set to a suitable length. By the tailpipe being set to a suitable length like this, it is possible to achieve a sufficient pulsation effect of the engine and thereby efficiently direct air into the exhaust muffler via the secondary air introduction pipe. Exhaust gas within the exhaust muffler is discharged to the outside through the tailpipe.
Further, one example of such engines equipped with a secondary air supply device is disclosed in Japanese Patent No. 2604659 (hereinafter referred to as “JP '659”), in which respective upper portions of a crankcase and a cylinder are positioned at generally the same height, a fuel tank is provided over a crankcase and an exhaust muffler is provided over a cylinder. With the exhaust muffler provided over the cylinder, the exhaust muffler is located at a high position so that a sufficient space is secured beneath the exhaust muffler. The tailpipe is provided to extend downward using the space, so that a suitable length of the tailpipe can be secured.
The engine equipped with a secondary air supply device disclosed in JP '659 is used, for example, in a power generator. In the case where such an engine is used in a power generator, the engine tends to be driven for a long time, and thus, it is necessary to secure a large storage capacity of the fuel tank. Therefore, in the case where the engine of a small size is used in a power generator, it is conceivable to secure a large storage capacity of the fuel tank by providing the fuel tank over both of the crankcase and the cylinder.
With the fuel tank provided over both of the crankcase and the cylinder as noted above, however, it is difficult to provide the exhaust muffler over the cylinder. To avoid such a difficulty, it is conceivable to provide the exhaust muffler laterally of the cylinder. In the case where the exhaust muffler is disposed laterally of the cylinder, the exhaust muffler is located at a low position. Therefore, it is difficult to secure a space sufficient to allow the tailpipe to extend beneath the exhaust muffler, so that the tailpipe cannot extend downward so as to have a sufficient length.